1. Field of the Invention
The present invention relates to a zoom lens system and further relates, though not exclusively, to an image pickup apparatus including the zoom lens system.
2. Description of the Related Art
Today, an image pickup apparatus using a solid-state image pickup device is becoming more sophisticated and compact. Examples of such an image pickup apparatus include a video camera, a digital still camera, a broadcasting camera, and a still camera using a silver salt film.
Accordingly, there is a demand for a compact zoom lens system which can be used as a photographic optical system for such an image pickup apparatus and has a short overall lens length while being capable of providing a high zoom ratio.
A four-unit zoom lens system is known as a compact zoom lens system having a high zoom ratio. The four-unit zoom lens system includes, in order from an object side to an image side, four lens units of positive, negative, positive, and positive refractive power and performs zooming by moving these lens units (see, for example, Japanese Patent Laid-Open No. 3-296706, U.S. Pat. No. 6,975,461, and Japanese Patent Laid-Open No. 2004-94233).
Also, a four-unit zoom lens system including a second lens unit composed of a pair of negative and positive lens elements is known (see, for example, U.S. Pat. No. 6,069,743 and U.S. Pat. No. 6,577,450).
To achieve both compactness of a camera and a higher zoom ratio of a zoom lens system, a retractable zoom lens system is used these days. The retractable zoom lens system refers to a zoom lens system in which, in a non-use mode (non-photographing mode), distances between adjacent lens units are reduced to distances that are different from those in a photographing mode. This can reduce the amount of protrusion of lens elements from the camera body.
Generally, as the number of lens elements included in each lens unit of a zoom lens system increases, the length of each lens unit on the optical axis increases.
At the same time, as the amount of movement of each lens unit for zooming or focusing increases, the overall lens length increases. This results in an increased length of the retractable barrel.
It is important for a zoom lens system for image pickup apparatuses to reduce the size of the entire lens system while providing a predetermined zoom ratio and good optical performance throughout the entire zoom range. Therefore, one can appropriately define the movement of each lens unit for zooming, the refractive power of each lens unit, and the lens configuration of each lens unit. For example, increasing the refractive power of each lens unit can reduce the overall lens length, as the amount of movement of each lens unit during zooming is reduced. However, simply increasing the refractive power of each lens unit causes larger variations in aberration associated with zooming, and makes it difficult to effectively correct the aberration.
In other words, while there is a trade-off between compactness and good optical performance, it is important for the zoom lens system to achieve both.